The support for dualboot was integrated on meta-digi-dualboot layer, but it
really depends only on environment variable 'dualboot' so we'd better
integrate the support on the scripts in meta-digi, to avoid synchonization
problems between both layers.
This also allows to be able to easily enable dualboot in U-Boot with the
variable, without needing to update the script on the linux partition.
Signed-off-by: Hector Palacios <hector.palacios@digi.com>